1. Field of the Invention
The present invention relates to silyl-functionalized polymers exhibiting increased stability toward atmospheric moisture. More particularly, this invention is related to a process for the preparation of silylated polyurethane polymers which avoids the use of tin catalysts in favor of alternative reduced toxicity catalysts.
2. Description of Related Art
Moisture-curable formulations are well known, particularly those which are based on silyl-functionalized polymers. The silyl-functionalized polymers may be prepared by a number of methods. One example is the reaction of polymers having end groups containing active hydrogen with isocyanates, particularly isocyanatoalkylalkoxysilanes. Another example is the reaction of isocyanate-terminated polyurethane polymers with aminosilanes or mercaptosilanes. The reaction is often carried out with urethane reaction-forming catalysts, i.e., catalysts that promote the isocyanate reaction with active hydrogen-containing compounds.
Known urethane reaction-forming catalysts include, for example, dialkyltin compounds, metal complexes (chelates and carboxylates) of bismuth and zinc and tertiary amine compounds.
A disadvantage of many known urethane reaction-forming catalysts for preparing silane-crosslinkable polyurethane polymers is that these polymers need to be handled under conditions which exclude moisture. In industrial practice this is associated with a high degree of complexity thus adding to the cost of manufacture.
Moreover, environmental regulatory agencies and directives have increased, or are expected to increase, restrictions on the use of tin-containing compounds in formulated products. For example, while formulations with greater than 0.5 weight percent dibutyltin presently require labeling as toxic with reproductive 1B classification, dibutyltin-containing formulations are proposed to be completely phased out in consumer applications over the next 4-6 years.
Accordingly, it is an object of the present invention to provide a process for the preparation of silylated polyurethane polymers employing non-tin urethane reaction-forming catalysts.